Description of the Prior Art
U.S. Pat. No. 3,857,247 to Lindsey J. Phares discloses an offshore tower which is fastened to a sea bed by means of anchor piles. These piles are driven down into the sea bed through tubular sleeves which are welded or otherwise affixed to the bottom of the tower. After the anchor piles have been driven, their upper ends, which are inside the sleeves, are locked to the sleeves by pumping grout or similar material down into the annular clearance between the pile and the sleeve. The grout then hardens to transfer loading stresses from the sleeve to the pile.
In order to be certain that the grout connection between the pile and the sleeve is complete and secure with an adequate margin of safety, it has been the practice in the art to provide elongated sleeves, e.g., more than 100 feet (30 meters) long so that a large area is available for grout interconnection. However, this proves to be quite costly. In addition, steel bars or rods, known as "shear connectors" are often welded to adjacent sleeve and pile surfaces to serve as keys for enhancing the grout locking action.
The use of grout for locking anchor piles to sleeves in depths of as much as 500 to 600 feet, has been quite difficult to carry out in a reliable manner because the grout has to be pumped over a great distance and there is no reliable way of ascertaining whether the grout has fully filled the space between each sleeve and its associated anchor pile.
It is important in anchoring an offshore tower to provide a positive locking action not only against downward loading imposed by the weight of the tower, but also to resist lateral loading and upward loading caused by the effects of wind, waves and water currents on the tower. These various effects, moreover are changeable and sporadic; and thus the locking arrangements must be capable of operating to prevent relative movement in different directions and they must not be affected by sudden strains and shocks. Also they must be capable of sustaining this locking effect over long periods of time, e.g., forty years, without maintenance.
Various other interconnecting arrangements have been employed in the prior art for locking an elongated member inside a sleeve, but none of them were capable of providing bi-directional locking in a reliable manner. One prior art locking arrangement is shown in U.S. Pat. No. 2,784,015 to C. G. Swanson. This patent shows a tubular outer sleeve-like member which is locked to an inner elongated member by means of two sets of wedges inserted between tapered facing surfaces of the elongated member and the sleeve. The wedges of the two sets act in mutually opposite directions to provide a bi-directional lock and they are held in locking engagement by means of tension rods which extend between the wedges of each group.
The Swanson wedge arrangement is unsuitable for long term locking of a sleeve to an elongated member where the elongated member is subjected to variously applied stresses in different directions. This is because the tension members of Swanson are subject to stretching from the long term effects of bending and stretching caused by wind or other elements acting on the sleeve and elongated member. As a result the forces holding the wedges in locked condition are not reliably maintained. Also, the Swanson wedges must be assembled by a workman working directly with them. They are not suited for installation at large water depths, e.g., 500 to 600 feet (150-180 meters) by operations carried out from above the surface of the water.